In the field of silver halide photographic light sensitive materials, it is common practice to colour photographic emulsion layers and other hydrophilic colloid layers so as to absorb light of specific wavelengths.
Medical x-ray films are often constructed with two light sensitive layers, one on either side of a transparent base. The film is imaged by phosphor screens placed on either side. It is undesirable for the light emitted from one screen to cross the base and expose the non-adjacent light sensitive layer. Such cross-over will lead to a blurred image, poor resolution and loss of image quality. This cross-over light gives the light sensitive layer an additional exposure and an apparent higher speed. The problems associated with cross-over can be eliminated by the incorporation of coloured hydrophilic colloid layers between the light sensitive layer and the base. Ideally, the dye used to colour these layers should have the following properties:
1) It should have good spectral absorption in the area of the spectrum where the phosphor screens emit (e.g., 545 nm with green screens). PA1 2) It should be photographically inert. PA1 3) It should be decolourised by the photographic processing solution and should not leave any undesirable colours on the material after processing. PA1 4) It should not diffuse from the coloured layer into the other layers. PA1 5) It should be stable in the photographic material for long periods of time before processing. PA1 each Y independently represents an electron attracting group, and PA1 each X represents a water solubilising group. PA1 R represents a substituted or unsubstituted alkyl group having from 1 to 4 carbon atoms PA1 Y is preferably COOR, COR, CF.sub.3 or CN etc., in which R is as defined above PA1 X is preferably SO.sub.3 H or a salt thereof. PA1 HOSTAPUR--wetting agent available from Hoechst (10% aqueous solution). PA1 DEXTRAN 40--polysaccharide available from Fisons. PEA--poly(ethyl acrylate) (aqueous dispersion). ##STR4## XP505--conventional X-ray film processor, available from Minnesota Mining and Manufacturing Company. PA1 XAD2--conventional X-ray film processing chemistry, available from Minnesota Mining and Manufacturing Company.
Item (4) is particularly important for medical x-ray materials since the dye-containing layer is adjacent to the light-sensitive layer. Diffusion of the dye into the light-sensitive layer can give rise to adverse effects. The coloured colloid layer must be coated at the same time as the light sensitive layer to minimise manufacturing costs. During this process, the light-sensitive photographic emulsion layer is in contact with the dye containing layer in the wet state. Diffusion of dye between these layers is very dependent on the structure of the dye.
A number of suitable dyes have been proposed in the art. Particulate dyes, mordants with dyes, and oil dispersed dyes have been used, but these present problems with simultaneous coating with the photographic emulsion layer.
U.S. Pat. No. 4,925,782 discloses dimeric merocyanine dyes for dyeing specific colloid layers in photographic light-sensitive elements. General formula (II) describes dimeric merocyanine dyes based upon N,N'-bisphenylpiperazine. None of the dyes exemplified based on this nucleus absorb green light. Of the other dyes exemplified in U.S. Pat. No. 4,925,782, none have an absorption maximum in the colloid layer close to 545 nm which is ideal for use in medical x-ray film. Exemplified dyes which absorb green light are a monomethine merocyanine (2-10) of .lambda..sub.max 528 nm and three trimethine merocyanines (1-49, 1-52 and 1-55) with, respectively .lambda..sub.max 563 nm, 575 nm and 573 nm.
Item (3) is also very important for medical x-ray films as incomplete bleaching of the dye will result in an increase in Dmin. For medical x-ray elements, the dye must be completely bleached during standard x-ray processing (90 seconds dry to dry). Dyes 1-49, 1-52 and 1-55 of U.S. Pat. No. 4,925,782 only are bleached to a residual colour of 0.01 (0.02 for double sided) following development for 3 minutes.
The dyes described in the state of the art do not satisfy all of the above mentioned 5 properties for medical x-ray films. There is a requirement for a non-diffusing dye with maximum absorption at about 545 nm that is bleached totally by photographic developer.